A typical wireless network includes a number of base stations each radiating to provide coverage in which to serve user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped devices, whether or not user operated. In turn, each base station may be coupled with a switch or gateway that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs served by the network.
Further, such a network may operate in accordance with a particular radio access protocol, examples of which include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA) (e.g., Long Term Evolution (LTE) and Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communications (GSM), IEEE 802.11 (WiFi), BLUETOOTH, and others. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between base station coverage areas, and other functions.
Each base station may provide wireless service to UEs on one or more carrier frequencies (carriers), each of which could be frequency division duplex (FDD), defining separate frequency channels for downlink and uplink communication, or time division duplex (TDD), defining a frequency channel multiplexed over time between downlink and uplink use. Each carrier or its respective channels could be within a defined frequency band and could be of a particular frequency bandwidth, such as 5 MHz, 10 MHz, or 20 MHz for instance, defining a certain extent of air interface resources. A given base station could be arranged to serve a UE on a single such carrier at a time or, with carrier aggregation service or the like, on multiple such carriers at a time.
Further, each base station in such a network may be communicatively linked with a signaling controller that carries out various network control functions, such as managing setup of bearer connections between the base station and one or more transport networks, tracking where UEs are located in the network, paging UEs, and the like. In addition, neighboring base stations may be communicatively linked with each other to facilitate handover and other inter-base station signaling.
By way of example, in an LTE network, each base station (LTE evolved Node-B (eNodeB)) has a communication interface with a signaling controller known as a mobility management entity (MME). The base station and MME each also have a respective communication interface with a gateway system that provides connectivity with a packet-switched transport network, and the base station has a communication interface with each of its neighboring base stations. Typically, the nodes of such an LTE network would sit on a wireless service provider's core packet-switched network (e.g., a network compliant with the industry standard system architecture evolution (SAE) for the LTE protocol), and so the base station and each other network entity (e.g., MME, gateway, and neighboring base station) may each have an assigned Internet Protocol (IP) address on that network, and the interfaces between these entities may be defined as logical connections (e.g., established virtual tunnels) through that network.
In example operation, when a UE enters into coverage of an LTE base station on a particular carrier, the UE signals to the base station to initiate an attach process and to establish a radio-link-layer connection with the base station. In this process, the base station signals to the MME, the MME authenticates the UE, the MME and base station obtain and store a context/profile record for the UE, and the gateway system assigns an IP address to the UE for use by the UE to communicate on the packet-switched transport network. Further, at this point or later, the MME may engage in signaling with the base station and the gateway system to establish for the UE one or more bearers for carrying packet data between the UE and the transport network. Each such bearer may have an associated quality of service (QoS) level indicated by a QoS class identifier (QCI) value, and packets transmitted on a given bearer could be tagged with the QCI value or a corresponding differentiated services code point (DSCP) value, so that network entities can route and otherwise handle the packets with an appropriate QoS level (e.g., with appropriate routing priority, etc.)
Once a UE is so attached with a base station, the base station then serves the UE on one or more carriers, managing downlink communication of packet data to the UE and uplink communication of packet data from the UE. For example, as the gateway system receives packet data destined to the UE, the gateway system may forward the packet data to the base station, and the base station may schedule and provide transmission of that data to the UE on the UE's serving carriers. Likewise, as the UE has packet data to transmit on the transport network, the UE may transmit a scheduling request to the base station, the base station may schedule transmission of that data from the UE on the UE's serving carriers, the UE may accordingly transmit the data to the base station, and the base station may then forward the data to the gateway system for output on the transport network.
Optimally, a wireless service provider will strategically implement base stations throughout a market area so that served UEs can move between the base station coverage areas without loss of coverage. Each base station may include an antenna structure and associated equipment, and the wireless service provider may connect the base station by a landline cable (e.g., a T1 line) with the service provider's network infrastructure to enable the base station to communicate with a signaling controller (e.g., MME), gateway system, other base stations, and the like.
In practice, however, it may be impractical for a wireless service provider to run landline connections to base stations in certain locations. For instance, where a service provider seeks to provide many small coverage areas blanketing a market area or to fill in coverage holes between coverage of other base stations, the service provider may implement many small-cell base stations throughout the market area, but it may be inefficient or undesirable to run landline cables to every one of those small-cell base stations.
To connect a base station with the network infrastructure in such a situation, the wireless service provider may implement a wireless backhaul connection between the base station and another base station of the service provider's network. In this situation, the base station at issue operates as a relay base station, and the other base station operates as a donor base station. In practice, the relay base station includes or is coupled (e.g., via a local area network or other connection) with a UE, referred to as a relay-UE, and the donor base station then serves the relay-UE in much the same way that the donor base station serves other UEs. Further, the relay base station itself serves UEs, in much the same way that any base station would.
With this arrangement, when the relay-UE attaches with the donor base station, the relay-UE may acquire connectivity and an IP address as discussed above for instance. But based on a profile record for the relay-UE, the network (e.g., a signaling controller) may recognize that the relay-UE is a relay-UE (rather than a normal end-user UE) and may therefore set up a bearer connection for that relay-UE with a special gateway system that provides for internal core network connectivity and assigns the relay-UE with an IP address for use to communicate within the core network. Once the relay-UE receives that core network IP address assignment, the relay-UE may then convey that IP address to the relay base station for use by the relay base station as the relay base station's IP address on the core network. The relay base station may then operate as a full-fledged base station of the network, having IP-based interfaces with other core network entities (e.g., a signaling controller, a gateway system, and other base stations), albeit with those interfaces passing via the wireless backhaul connection provided by the relay-UE, and via the special gateway system.
Once the relay base station is thus in operation, the relay base station may then serve UEs in the same way as a standard base station serves UEs. Thus, when a UE enters into coverage of the relay base station, the UE may signal to the relay base station to initiate an attach process, the UE may acquire an IP address, and an MME may engage in signaling to establish one or more bearers between the UE and a gateway system. Each of these bearers, though, like the relay base station's signaling communication, would pass via the relay's wireless backhaul connection.
Moreover, when a UE is served by a base station, regardless of whether the base station is a relay base station or a standard base station, the UE may regularly monitor the reference signal from that base station and reference signals from other base stations in the vicinity, to help ensure that the UE continues to operate in a most appropriate coverage area (e.g., in a coverage area with the strongest signal strength). If the UE finds that one or more other base stations provide sufficiently strong coverage, perhaps sufficiently stronger than the UE's currently serving base station, then the UE may initiate handover. For instance, the UE may transmit to its serving base station a measurement report that specifies the one or more detected coverage areas and, for each such coverage area, the detected signal strength. The serving base station (source base station) and/or associated network infrastructure may then decide based on the UE's measurement report to process a handover of the UE to a particular base station (target base station) from which the UE detected sufficiently strong signal strength.
In an example handover process, the source base station may direct the UE to transition to be served by the target base station, and the UE may then engage in control channel signaling with the target base station in order to transition from having a radio-link-layer connection with the source base station to having a radio-link-layer connection with the target base station. For instance, in response to a radio-link-layer control message from the source base station, the UE may transmit to the target base station, on a random-access channel, an access request, and the target base station may receive that access request and transmit to the UE an access response. Once the UE receives that access response, the UE may then transmit to the target base station a radio-link-layer control message to complete transition of the UE from having a radio-link-layer connection with the source base station to having a radio-link-layer connection with the target base station.
When a UE is located near a border between neighboring base station coverage areas, the strength of signals from the base stations may fluctuate as a result of the UE physically changing locations or as a result of fluctuations in the signal strengths of the coverage areas due to variations in network load or other factors. Such signal strength fluctuations may cause the UE to “ping-pong” between being served by the neighboring base stations, during which the UE is repeatedly handed off from being served by one base station to being served by another. Each time the UE is handed off between base stations, the base stations may engage in inter-base station control channel signaling as described above.